Optical system in existing reflection type liquid crystal projectors is basically constituted by a color separating optical system for splitting illuminating light rays from a white light source into three RGB wavelength components, three reflection type liquid crystal display panels for reflecting the separated RGB wavelength components of the illuminating light to produce image light of the respective components, and a color integrating optical system for integrating the reflected image light from the respective liquid crystal display panels into a full-color image. For example, the color separating optical system is constituted by a combination of optical elements such as dichroic mirror and polarizing beam splitters. On the other hand, the color integrating optical system can be constituted by a combination of optical elements such as polarizing beam splitter and a dichroic prism. With regard to the optical systems of this sort, various proposal have thus far been made as disclosed, for example, in Laid-Open Japanese Patent Applications 2001-92005, 2001-100155 and H11-326861.
In the case of the reflection type liquid crystal projector mentioned above, the optical system employs a polarizing beam splitter or splitters as an optical device for separating illuminating light rays into three RGB wavelength components or for separating two wavelength components of illuminating light rays after separating one wavelength component by the use of a dichroic mirror. Further, a polarizing beam splitter and a dichroic prism are also used at the time of integrating image light of RGB wavelength components which are reflected by three reflection type liquid crystal display panels.
In this connection, the polarizing beam splitter and the dichroic prism have been generally considered as an optical device or element which is suitable for transmitting p-polarization light while reflecting s-polarization light. Of course, there are optical devices or elements which are geared to reflect p-polarization light while transmitting s-polarization light. Generally, a polarizing beam splitter or a dichroic prism, which has a polarizing film or a dichroic film of multiple layer construction, needs to have a more than two times greater number of layers to function as an optical element which reflects p-polarization light and transmits s-polarization light, as compared with a polarizing beam splitter or dichroic prism which is designed to transmit p-polarization light and reflect s-polarization light. Naturally, the greater the number of layers, the higher becomes the cost as an optical element. Nevertheless, in the optical arrangement according to the above-mentioned prior art, a polarizing beam splitter and a dichroic prism are partly used for reflection of p-polarization light and transmission of s-polarization light despite a great advantage that they make the whole optical system extremely expensive.
Of the above-mentioned prior art publications, Laid-Open Japanese Patent Application H11-326861 discloses a reflection type liquid crystal projector which is arranged to split illuminating light in the state of s-polarization light, firstly separating a green wavelength component of the illuminating light from blue and red wavelength components by the use of a dichroic mirror, and then passing the blue and red wavelength components through a half wave plate thereby to convert the red wavelength component to p-polarization light by rotating the plane of polarization by 90 degrees. The green wavelength component of the illuminating light, which is still in the state of s-polarized light, is reflected by one polarizing beam splitter toward a liquid crystal display panel. The p-polarized red component of the illuminating light and the s-polarized blue component of the illuminating light are separated by another polarizing beam splitter, which permits the p-polarized red component to pass through a polarizing surface while reflecting the s-polarized blue component by the polarizing surface. The separated red and blue components of the illuminating light are directed toward the respective liquid crystal display panels. Accordingly, one of the polarizing beam splitters in the optical system is required to reflect s-polarization light and transmit p-polarization light.
In the case of the optical system which is disclosed in Laid-Open Japanese Patent Application H11-326861, a dichroic prism is used in ultimately integrating image light of three wavelength components together. This dichroic prism needs to have characteristics of transmitting image light of p-polarized green wavelength component while reflecting image light of s-polarized red wavelength component and p-polarized blue wavelength component.
In this regard, it is important to note that transmission characteristics or transmittivity of the dichroic prism vary conspicuously depending upon the angle of incidence of input light. Normally, a reflection type liquid crystal projector is provided with a converging lens in its light source, so that a light flux from the light source is converged toward a dichroic prism to enter the latter with a certain angle of incidence. In a case where image light of the green wavelength component is transmitted through the dichroic prism as p-polarization light and image light of the red wavelength component is reflected by the dichroic prism as p-polarization light as in the above-described prior art, a conspicuous drop in transmittivity of image light of one wavelength component can occur in connection with its angle of incidence to invite degradations in image light output efficiency. This phenomenon makes it necessary to minimize the angle of incidence on the dichroic prism by using a less bright converging lens with a larger f value, although it will again lead to the problem of degradations in image light output efficiency.